I. Field of the Invention
The present invention relates generally to component assemblages for facilitating electromagnetic communication between a pair of coaxial transmission lines, at least one of which is adapted to rotate about a longitudinal axis. More particularly, the present invention relates to a novel component assemblage in which a waveguide structure is used to couple electromagnetic energy between the pair of coaxial transmission lines.
II. Description of the Related Art
In certain satellite communication systems, such as is described in U.S. Pat. No. 4,979,170, entitled ALTERNATING SEQUENTIAL HALF DUPLEX COMMUNICATION SYSTEM, which is assigned to the assignee of the present invention and is herein incorporated by reference, communication is maintained between a terrestrial control station and a large number of mobile units through satellite repeaters. Each mobile unit is equipped with a communications transceiver for processing information signals exchanged with various ones of the satellite repeaters. Since the orientation of each mobile unit transceiver relative to a given satellite repeater will vary as the mobile unit changes direction during terrestrial navigation, each mobile unit transceiver will preferably include a rotating antenna for providing omnidirectional signal transmission and reception. Rotation of the mobile unit antenna requires that a rotary joint be provided to enable connection of the mobile unit signal processing electronics to the rotating antenna.
Referring to FIG. 1, there is shown a cross-sectional view of a conventional rotary joint 10 operative to facilitate communication between a rotating coaxial transmission line 14 and a fixed coaxial transmission line 18. The transmission line 14 is coupled to an antenna (not shown), and includes an inner conductor 19 disposed to rotate together with the antenna about a vertical axis V. The fixed transmission line 18 is coupled to transmit/receive networks (not shown) within the mobile unit. As is indicated by FIG. 1, electromagnetic energy is coupled between the transmission lines 14 and 18 through a quarter-wavelength choke 22. That is, the choke 22 is selected to be of a length L equivalent to a quarter-wavelength (.lambda.,/4). As is well known, the choke 22 serves to provide coupling between the transmission lines 14 and 18.
The choke 22 is seen to include a small diameter quarter-wavelength finger 26 of the inner conductor 19 of the transmission line 14. The finger 26 is circumscribed by a radial cavity defined by a conductive segment 34 of the inner conductor 38 of transmission line 18. The choke 22 further includes a quarter wavelength tubular section 42 extending from a tubular outer conductor 46 of the transmission line 18. The quarter wavelength section 42 circumscribes a portion of an outer conductor 50 of transmission line 14.
Several characteristics of the choke 22 tend to disadvantage performance of the rotary joint 10. For example, the small diameter of the finger 26 renders this element susceptible to damage resulting from vibration accompanying terrestrial movement of the mobile unit. Similarly, precise tolerances are required to be maintained between the finger 26 and surrounding radial cavity defined by conductor 34, as well as between the quarter wavelength section 42 and the outer conductor 50 of transmission line 14. This increases manufacturing cost, and makes the joint 10 further susceptible to damage resulting from mechanical vibration. Finally, the .lambda./4 length of the choke 22 may be undesirable in applications requiring a rotary joint of relatively small axial dimension.
As is described hereinafter, the present invention provides a rotary coupling assemblage designed to obviate the disadvantages associated with conventional rotary joints.